The present disclosure relates to an image sensor and a method for manufacturing the same.
An image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor may be classified into a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
During the fabrication of image sensors, a photodiode may be formed in a substrate using ion implantation. As the size of a photodiode is reduced for the purpose of increasing the number of pixels without increasing chip size, the area of a light receiving portion is also reduced, thereby resulting in a reduction in image quality.
Also, since a stack height does not reduce as much as the reduction in the area of the light receiving portion, the number of photons incident to the light receiving portion is also reduced due to diffraction of light called Airy disk.
As an alternative to overcome this limitation, an attempt of forming a photodiode using amorphous silicon (Si), or forming a readout circuitry in a silicon (Si) substrate using a method such as wafer-to-wafer bonding, and forming a photodiode on and/or over the readout circuitry has been made (referred to as a three-dimensional (3D) image sensor).
In the related-art, the photodiode and the readout circuitry are connected through metal lines in the manufacturing of a 3D image sensor. A tungsten plug (W-plug) is formed over the metal lines prior to bonding the photodiode and the readout circuitry. However, a bonding area with the photodiode is reduced due to a CMP dishing generated in a tungsten CMP process, resulting in degradation of the bonding efficiency. Accordingly, there is a limitation in acquiring an ohmic contact due to a poor contact between the metal line of the readout circuitry and the photodiode.
Also, in the related-art, there is a limitation of weak bonding strength between a contact plug of a lower substrate including a readout circuitry and a metal of a photodiode.
Since both the source and the drain at sides of the transfer transistor in the related art are heavily doped with N-type impurities, a charge sharing phenomenon occurs. When the charge sharing phenomenon occurs, the sensitivity of an output image is reduced and an image error may be generated. Also, because a photo charge does not readily move between the photodiode and the readout circuitry, a dark current is generated and/or saturation and sensitivity is reduced.